First commissioning of the SuperKEKB vacuum system

Abstract

The first (Phase-1) commissioning of SuperKEKB, an asymmetric-energy electron-positron collider at KEK, began in February 2016, after more than five years of upgradation work on KEKB and successfully ended in June 2016. A major task of the Phase-1 commissioning was the vacuum scrubbing of new beam pipes in anticipation of a sufficiently long beam lifetime and low background noise in the next commissioning, prior to which a new particle detector will be installed. The pressure rise per unit beam current decreased steadily with increasing beam dose, as expected. Another important task was to check the stabilities of various new vacuum components at high beam currents of approximately 1 A. The temperature increases of the bellows chambers, gate valves, connection flanges, and so on were less than several degrees at 1 A, and no serious problems were found. The effectiveness of the antechambers and TiN coating in suppressing the electron-cloud effect (ECE) in the positron ring was also confirmed. However, the ECE in the Al-alloy bellows chambers was observed where TiN had not been coated. The use of permanent magnets to create an axial magnetic field of approximately 100 G successfully suppressed this effect. Pressure bursts accompanying beam losses were also frequently observed in the positron ring. This phenomenon is still under investigation, but it is likely caused by collisions between the circulating beams and dust particles, especially in the dipole magnet beam pipes.

SuperKEKB collider

“…A new collimator for antechamber beam pipes was developed based on those used in PEP II at SLAC. Two new collimators were installed in the LER and checked with a 1 A beam in Phase 1 commissioning [21]. Expected performance was confirmed, including low temperature rise in the bellows behind the moving collimator head and collimator head positioning accuracy; mass production was conducted to add more collimators for Phase 2.…”

“…To cure this problem, permanent magnets with yokes were attached to the bellows chambers to form an axial magnetic field. Then the ECE was effectively suppressed [21]. In drift space, even with antechambers and TiN coating, electron clouds still formed without longitudinal magnetic fields; hence, before Phase 2 commissioning, permanent magnets with yokes were installed in the drift space of all arc sections and the Tsukuba straight section for higher beam currents.…”

Mitigating the electron cloud effect in the SuperKEKB positron ring

“…One PM had a diameter of 30 mm and a magnetic field strength at the surface of approximately 85 mT. A B z value of approximately 10 mT was formed in most regions of the PM units, although the polarity reversed locally close to the PMs [60] (Fig. 7 in Sec.…”

“…In spite of the countermeasures described above, the ECE was first observed in phase-1 commissioning under a beam current (I [mA]) of approximately 600 mA at a bunch fill pattern of 1=1576=3.06RF [60,61]. The vertical beam size began to blow up from this value of I, as shown in Fig.…”

“…5. One type-1 PM unit had eight PMs attached to a Cshaped iron plate (yoke) [60]. One PM had a diameter of 30 mm and a magnetic field strength at the surface of approximately 85 mT.…”